These findings prompted researchers to undertake a new study to test the theory that not sleeping on the left side, and specifically sleeping on the back, would be associated with a higher risk of late stillbirth. Researchers also set out to investigate whether increased sleep duration and sleeping during the day would be linked with an increased risk of stillbirth.

The study involved 164 women who had experienced a stillbirth from at least 28 weeks of gestation. The control group consisted of 569 women with ongoing pregnancies.

All participants were interviewed face-to-face by research midwives and information was collected on going-to-sleep positions, sleep duration, and daytime sleep.

Researchers found an increase in late stillbirth risk among women who went to sleep on their backs and those who frequently changed position when settling to sleep and couldn’t remember their final sleep position.

There was also a borderline increase in risk among women who did not go to sleep on their left side.

The sleep-related risk factors for late stillbirth

After analyzing the data, researchers identified the sleep-related risk factors for late stillbirth as:

Going to sleep on the back (3.7-fold increased risk)

Less than six hours of sleep (1.8-fold increased risk)

These risks were found to be independent of other risk factors for late stillbirth such as obesity, smoking, high maternal age, and small gestational age.

Researchers did not find a link between daytime sleep or long sleep duration and late stillbirth risk.

Why does sleeping on your back increase stillbirth risk?

The authors of this study pointed out that research has found:

The size of the vein that carries blood from the lower half of the body to the heart is smaller in pregnant women who sleep on their backs compared to pregnant women who sleep on their left side.

Blood flow in the artery that supplies blood to the uterus is reduced when sleeping on the back, compared to sleeping on the left side.

Sleeping on the back appears to reduce oxygen supply to the fetus.

Researchers also pointed out that sleeping on the back is associated with sleep-disordered breathing — and this has been linked to pregnancy complications such as hypertensive disorders, gestational diabetes, and underweight babies.

The findings of this study led the authors to argue that public health education should be developed to encourage women in the third trimester to avoid sleeping on their backs, especially after 37 weeks of pregnancy — and that such an intervention could reduce late stillbirth by as much as nine percent.

How concerned should pregnant women be?

In an interview with HealthCentral, Professor Lesley McCowan, lead author of the study and professor of obstetrics and gynaecology at the University of Auckland, pointed out these findings are particularly important for women in the last three months of pregnancy — but that more research is needed to better understand the mechanisms for the effects reported by the study.

Should pregnant women try to modify their sleep position?

Professor McCowan suggested that pregnant women who prefer sleeping on their backs should try to fall asleep on their sides instead. She also stated that rolling onto the back during the night is relatively common and shouldn’t be something to worry about. If this happens, simply settle back to sleep on your side (a pillow placed behind the back can help prevent this from happening).

Professor McCowan recommended that pregnant women think about their sleep position in the third trimester since there is no evidence to suggest there is any increase in stillbirth risk before this stage of pregnancy. With that being said, she did point out the risk may be even greater at term compared to the third trimester — so it’s never too late to pay attention to your sleep position.

Martin is the creator of Insomnia Coach, an eight-week course that combines online sleep education with individual sleep coaching. His course helps clients improve their sleep so they can enjoy a better life with more energy and start each day feeling happy, healthy, rested, and refreshed. Martin also runs a free sleep training course that has helped over 5,000 insomniacs. He holds a master’s degree in health and wellness education and studied clinical sleep health at the University of Delaware.